This invention relates to a process for making a composite of a resin matrix reinforced with stretch broken fibers and the product thereof.
U.S. Patent No. 4,759,985, granted Jul. 26, 1988, of common assignee discloses a variety of methods for forming a composite of a resin matrix reinforced with stretch broken fibers such as winding a stretch broken sliver on a frame covered with a film of thermoplastic resin to form a warp. The warp of stretch-broken sliver, however, can be made by any technique known to those skilled in the art, e.g., by creeling or beaming. A preform is obtained when another film of thermoplastic resin is placed over the warp to form a sandwich which is heated in a vacuum bag and then removed from the frame. Several of such preforms may be stacked while offset to provide multi-directionality and then the stack may be heated under pressure to form a composite structure.
Other techniques for applying matrix polymer include sprinkling of powdered resin on the sliver warp followed by heating to melt the resin, flowing liquid resin over the sliver warp, intermingling thermoplastic fiber with the sliver warp and then heating to melt the thermoplastic fiber thereby forming the matrix resin, calendering the warp between layers of matrix film, etc.
The composites formed are useful for deep drawing purposes with little sacrifice of strength and stiffness as compared to composites formed from resin reinforced with continuous filaments. However, in U.S. Pat. No. 4,759,985, the fibers do translate considerable distances. Draft ratios for this process typically run 200-300%. This high degree of translation creates the chance for alignment to be lost and a waviness becomes apparent on the surface of the tow. Finishes are applied to the yarn to reduce static, and to provide some cohesiveness to keep filaments from flying away. But, finish acts only as a weak damper for the energy released by the fiber when it breaks; much of this recoil energy is eventually dissipated by fiber movement resulting in misalignment.